CH-RD. S2 Packaging & Reliability Power and productivity

for a better world TM abb

Motivation & Objectives Optimization of TLP Emitter Bonding

Theory & Model

Bonding technologies play an indispensable role in the assembly of power electronic (PE)

modules. With increased requirements for higher operation junction temperatures, longer life-

time and environmental considerations, transient liquid phase (TLP) bonding is becoming a

possible promising alternative packaging technology.

References: [1]. L. Feller et al., "Lifetime Analysis of Solder Joints in High Power IGBT Modules for Incresing the Reliablity for Operation at 150°C," 2009. [2]. O. Grant et al., "Overview of transient liquid phase and partial transient liquid phase

bonding," Journal of Materials Science, no. 5305-5323, p. 46, 2011. [3]. L. Deillon et al., "Growth of Intermetallic Compounds in the Au-In System: Experimental Study and 1D modelling," Acta Materialia, 2012.

Transient Liquid Phase Bonding

for Power Electronic Modules Quanbo Jiang

1, Franziska Brem

2, Chunlei Liu

2, Michel Rappaz

1

1Institute of Materials Science and Engineering, Swiss Federal Institute of Technology Lausanne (EPFL), 1015 Lausanne, Switzerland.

2ABB Swiss Corporate Research Center (CHCRC), 5405 Baden-Dättwil, Switzerland.

Contact: Quanbo Jiang via quanbo.jiang@epfl.ch

Conclusions & Outlook

Acknowledgements

AlSiC Baseplate

Substrate (ceramic insulator)

Metallization

Metallization

Solder

Chip

Silicone

gel Plastic

cover

Gate Al bond wire

Cooler

Transient liquid phase (TLP) bonding

is a process for joining components

via the dissolution of a solid-state

material into a liquid-state interlayer

and the formation of intermetallic

compounds (IMCs) by interdiffusion

of materials[2].

Assumptions of the model[3]:

Chemical potential varies linearly through

the IMC phases.

Sn is initially saturated with Cu.

No nucleation stage occurs.

Local equilibrium reached at every interface.

Gradient of chemical potential:

𝐽 = −𝑀𝐶𝜕µ

𝜕𝑥

Fick’s first diffusion law:

𝐽 = −𝐷𝜕𝐶

𝜕𝑥

Fick’s second law:

𝜕𝐶

𝜕𝑡= 𝐷

𝜕2𝐶

𝜕𝑥2

Arrhenius equation:

𝐷𝐵 = 𝐷𝐵0𝑒(−

𝐸𝑎,𝐵𝑅𝑇 )

Velocity of interface:

𝑣𝛼/𝛽 =𝑑𝑥

𝑑𝑡=

1

(𝐶𝐵𝑏−𝐶𝐵

𝑎)𝐷 𝛼

𝜕𝐶𝐵𝑎

𝜕𝑥− 𝐷 𝛽

𝜕𝐶𝐵𝑏

𝜕𝑥

𝐶𝑢6𝑆𝑛5 (𝜂) 𝐶𝑢3𝑆𝑛 (𝜀)

𝐶𝑢 𝑆𝑛

𝑆𝑛 𝜺 𝜼 𝐶𝑢 𝐶𝑆𝑛 𝜇

𝜇2

𝜇3

𝜇1

𝐶𝑆𝑛𝑐𝑢

𝐶𝑆𝑛𝑆𝑛

𝑣1∗ 𝑣2

∗

𝑣3∗

Parameters:

𝐽 : diffusive flux

𝑀 : atomic mobility

𝐶 : Concentration

µ : Chemical potential

𝐷 : Diffusion coefficient

𝐸𝑎,𝐵 : Activation energy

𝑣𝛼/𝛽: Velocity of interface

Supporting Experiment for Model

Variac Thermocouple

Ar flow

He flow

Chip

Chip

Top Metal Plate Leadframe

Motivation of leadframe emitter bonding by TLP:

Replace the wire bonds with full area bonds.

Improve for higher power density and higher

junction temperature.

Reduce the inductance.

Objectives:

Develop 1D finite model for the

growth of intermetallic

compounds (IMCs).

Determine diffusion coefficients

in Cu/Sn system.

Optimize process parameters for

TLP leadframe emitter bonding.

Reduce void percentage of the

bonding areas.

Analyse and improve the

properties of resulting joints.

Schematic structure of high power module package[1].

Si chip

Cu lead frame

Galvanic Cu

Sn foil

Cu metallization

Cu metallization AlN substrate

Si chip

Cu lead frame

Program profile

Pgm2:

Pgm2:

real

temperature

profile

Pgm3:

Pgm3:

real

temperature

profile

Pgm4:

Pgm4:

real

temperature

profile

cycles VP_L

(%) B1_S2

VP_R

(%)

VP_L

(%) B2_S2

VP_R

(%)

0 1.6 1.5 2.7 3.9

200 broken 5.6 10.8 24.5

400 broken 7.0 14.9 34.0

800 broken 17.1 33.2 broken

1000 broken 21.5 31.0 broken

0

10

20

30

40

50

60

70

80

90

100

0 500 1000

Vo

id p

erce

nta

ge (

%)

Cycles (time)

B1_S2_L

B1_S2_R

B2_S2_L

B2_S2_R

Properties Cu6Sn5 Cu3Sn Copper

Melting point (°C) 415 676 1083

Density (𝑔 𝑐𝑚3 ) 8.28 8.90 8.94

Hardness (HV) 417±36 343±12 155±20

Shear strength (MPa) 32.9±10.9 210.0

0

20

40

60

0 5 10 15 20 25

Shea

r st

ren

gth

(M

Pa)

Void percentage (%)

Shear strength (Mpa) VS Void percentage (%)

The work presented in this thesis was conducted at ABB CHCRC, Baden and LSMX, Lausanne. I

first would like to thank my supervisors, Prof. M. Rappaz, Dr. F. Brem and Dr. C. Liu, for their kind

encouragement and patient guidance throughout my thesis work. Furthermore, I would like to

thank Dr. L. Deillon for her help with the model and experiment at LSMX. Moreover, I am very

grateful to all the employees at ABB for cultivating a friendly and motivating working environment.

The schematic diagram of experimental

furnace for dipping Cu wire inside Sn bath is

shown above. The growth of IMCs was

measured at liquid state (300°C) and at solid

state (200°C) over time.

@300°C.

@200°C.

400s.

3600s.

6400s.

1h.

9h.

100h.

Factors of Sn in phases 𝐷0(𝑚2/𝑠) 𝐸𝑎(kJ/mol)

𝐶𝑢 5.1 × 10−9 103.5

𝐶𝑢3𝑆𝑛 4.1 × 10−8 92.0

𝐶𝑢6𝑆𝑛5 8.7 × 10−9 78.8

D of Sn in phases D @300°C(𝑚2/𝑠) D @200°C(𝑚2/𝑠)

𝐶𝑢6𝑆𝑛5 6.0 × 10−16 1.8 × 10−17

𝐶𝑢3𝑆𝑛 1.8 × 10−16 3.0 × 10−18

𝐶𝑢 2.0 × 10−18 2.0 × 10−20

The growth kinetics of IMCs (above) follows

parabolic law. The diffusion coefficients

(below) were obtained from the simulation by

fitting to the experimental results. The

temperature dependency was determined.

Advantages and disadvantages of TLP

emitter bonding process for PE modules:

+ The Cu-Sn system is suitable for

packaging of power electronic modules

and low void percentage joints can be

achieved.

+ Low pressure is applied for clamping the

joining parts without damaging chips.

+ Higher mechanical strength and melting

point than Sn and soft solders are reached.

+ Lower cost for raw materials than SAC

solder and other new bonding materials.

- High requirement for the surface finish,

flatness and cleaning process of joining

partners.

- Long process times with vacuum furnace.

- Uniform bond line thickness is difficult to

achieve.

Process parameters were improved for TLP

process during 6-month work at ABB

CHCRC. The results demonstrate that TLP

emitter bonding by is a promising and

reproducible process for replacing the wire

bonding and for other small-area bonding.

Si

½ hard Cu

Sputtered

Cu

𝐶𝑢3𝑆𝑛

𝐶𝑢3𝑆𝑛

𝐶𝑢6𝑆𝑛5 Sn

𝐶𝑢3𝑆𝑛

𝐶𝑢6𝑆𝑛5 𝐶𝑢3𝑆𝑛 Galvanic

Cu

Initial setting parameters in the furnace

Pgm1 2h holding time @320°C, vacuum @230°C, inner water cooling

Pgm2 2h holding time @320°C, vacuum @260°C, N2 cooling

Pgm3 2h holding time @320°C, vacuum @320°C, 5min waiting time, N2

pgm4 2h holding time @320°C, vacuum @240°C and start H2 flow, N2

The TLP process parameters for leadframe

emitter bonding including load on the chips,

heating ramp, and vacuum steps were

optimized. Cu joining partners with different

surface roughness, grain size and thickness

range were compared.

SAM, SEM, EDX, hardness, thermal shock

cycling and shear tests were used for

characterizing the joint properties.

Optimal

Results:

Pgm3

(profile)

Thinner

Galvanic Cu

plating

0.06MPa

(load)

Si

½ hard Cu

Sputtered Cu

Galvanic Cu

Program 2 Program 4

280°C 300°C

Cu fully comsumed,

Sn left

Residual Sn

0.02MPa 0.04MPa 0.06MPa 0.08MPa

Batch Number B7_G1_Left B8_S1_Left B9_G2_Left B11_G4_Left

Optical

Microscope

SAM images

Void

percentage best

samples (%)

26.4 2.0 1.9 2.4

Average void

percentage (%) 35.6 11.8 8.8 7.8

pressed-

out Sn

pressed-

out Sn

020406080

100120140160180200220240260280300

0 15 30 45 60 75 90 105 120 135 150 165 180 195 210

Tem

pe

ratu

re (°

C)

Time (min)

020406080

100120140160180200220240260280300320

0 15 30 45 60 75 90 105 120 135 150 165 180 195 210

Tem

pe

ratu

re (°

C)

Time (min)

020406080

100120140160180200220240260280300320

0 15 30 45 60 75 90 105 120 135 150 165 180 195 210

Tem

pe

ratu

re (°

C)

Time (min)

Impact of void percentages for thermal

shock resistance and shear strength.